This invention relates to a dental handpiece on which a tool for dental treatment can be mounted detachably. More particularly, it relates to a dental handpiece having a rotatable contact member capable of preventing mechanical or thermal injuries otherwise caused by mistaken detachment operation of the dental treatment tool mounted thereon.
Up to now, a dental handpiece having a dental treatment tool mounted thereon has been used extensively for dental treatment. Such conventional dental handpiece is now explained by referring to FIG. 4.
FIG. 4 shows the distal end part of a conventional dental handpiece 40 which is made up of a head casing 41 for mounting a dental treatment tool 9 thereon and a head cap 42 threadedly attached to an upper part of the head casing 41.
Within the interior of the head casing 41, there is detachably mounted an inner casing 54 made up of a cartridge casing 54a and a casing cap 54b. The cartridge casing 54a has various components for holding and rotating the dental treatment tool 9 as later explained. Within the inner casing 54, there are arranged a rotary shaft 44 having a collet 43 detachably holding the dental treatment tool 9 and ball bearings 45, 46 for rotatably supporting the rotary shaft 44.
The collet 43 has plural axially extending slits 43a to permit the inner periphery at its lower end to be enlarged in diameter. The outer periphery of the collet 43 presents a tapered surface 55, the diameter of which is progressively increased towards the lower side. The collet 43 has its upper end projected above the rotary shaft 44 and formed with female screw threads. A spring 47 is placed around the outer periphery of the upper end of the collet 43. The upper tapped end of the collet 43 is engaged with a flange 49 having male screw threads 48.
The rotary shaft 44 is tubular and is formed at its lower end with a tool receiving opening 44a for receiving the dental treatment tool 9 therein. A rotor 44b in the form of an impeller is mounted on the outer periphery of the rotary shaft 44 for rotationally driving the rotary shaft 44. The rotary shaft 44 has its inner surface formed as a tapered surface 55 shaped for mating with the outer periphery of the collet 44. By such tapered inner peripheral surface of the rotary shaft 44, the collet 43 biased by the spring 47 in an upper direction, is pressed and held by the rotary shaft 44.
The ball bearings 45, 46 are arranged at the upper and lower portions of the rotary shaft 44, and are engaged with and retained by the head casing 41 and the head cap 42 via dampers 50, 51, respectively.
The head cap 42 is threadedly engaged with an upper part of the head casing 41 and has at its mid portion a pushbutton 52 for attachment and detachment of the dental treatment tool 9 held by the collet 43. The pushbutton 52 also has an engagement portion 52a capable of being engaged with a mating engagement portion 42a of the head cap 42. The pushbutton 52 has at its lower surface a spring 53 for upwardly biasing the pushbutton 52.
With the above-described dental handpiece 40, the rotor 44b provided on the rotary shaft 44 runs the dental treatment tool 9 held by the collet 43 in rotation by compressed air supplied from an air supply conduit, not shown.
For detachment of the dental treatment tool 9 from the dental handpiece 40, the pushbutton 52 is pressed down for lowering the collet 43 in order to permit the lower tapered surface 55 of the collet 43 to be enlarged so as to assure facilitated detachment of the dental treatment tool 9.
However, with such detachment mechanism of the conventional dental handpiece 40, the pushbutton 52 may be contacted with the flange 49 during the operation of the dental treatment tool 9, even if the pushbutton 52 is not pressed down so strongly as to extract the dental treatment tool 9. Thus, it may occur that, during treatment of a molar, above all, cutting of a narrow side portion, or preparation of a tooth base for a crown or a bridge, the pushbutton 52 may be caused to bear against some portion in the oral cavity, thereby inadvertently thrusting the pushbutton 52 downwards.
At this time, the pushbutton 52 thus thrust inadvertently tends to be contacted with the rotating flange 49, thus generating frictional heat and instantaneously producing a high temperature and hence an extremely dangerous state.